Post Tagged with: "anti-cancer"

In a recent discussion (Nicolau), about the suggested move of Prof. NicoIau from Scripps, the issue of the practicality of natural product total synthesis was raised. Here is a wonderful example of just that very usefulness, a wonderful piece of science extending over many years. It concerns the journey from Halichondrin B to Eribulin (E7389) a novel anti-cancer drug. The two compounds have the following structures:

I think you can see the relationship and as a development chemist I am glad they managed to simplify things (a bit).

Both compounds have an enormous number of possible isomers: Halichondrin B, with 32 stereocenters has 232 possible isomers; Eribulin has 19 with 219 isomers (if I have counted correctly, it does not really matter, there are lots of isomers). Remarkable is the fact that only one of these isomers is active in the given area of anti-cancer agents.

An excellent review of the biology and chemistry of these compounds has been published by Phillips etal1. This review is an excellent read and is to be commended. Another one written by Kishi2, is also full of information about the discovery of E7389 and I hope you will all get a chance to read this chapter.

The history of Halichondrin B goes back to 1987 when Blunt2-5 isolated it with other similar compounds from extraction of 200Kg of a sponge. Independently Pettit isolated the same compound from a different species4. The appearance of this compound in different species of sponge may indicate that it is produced by a symbiote.

The biological activity of Halichondrin B is amazing. When evaluated against B-16 melanoma cells it was found to have an IC50 of 0.093ng/mL. Against various cancers, generated in mice, it was shown to be affective at a daily dose of 5ug/kg, which resulted in a doubling of the survival rate. It has also been demonstrated that Halichondrin acts as a microtubule destabiliser and mitoitic spindle poison. It was proven that it is has tremendous in vivo activity against a variety of drug resistant cancers, lung, colon, breast, ovarian to mention a few. Consequently the National Cancer Institute selected it for pre-clinical trials and it’s here that the problems began. According to reference 1 the entire clinical development would require some 10g, and if successful the annual production amount would be between 1-5 kg. Blunt and co-workers managed to isolate 310mg from 1000kg-harvested sponge therefore, the only way to obtain the amounts required is total chemical synthesis. But synthesising 1-5 kg of such a compound would indeed be a mammoth task.

Kishi synthesised this compound7 in 1992 starting from carbohydrate precursors employing the Nozaki-Hiyama-Kishi Ni/Cr reaction, several times, in the long synthetic sequence8, 9. Now as an aside I have used this reaction on scale several times and although it works well its success is very dependant upon the quality of the chromium source and also the presence of other trace transition metals.

In collaboration with Eisai work on the SAR of Halichondrin began. They had a good start: Thanks to the total syntheses of Kishi several advanced intermediates were available for biological screening and one popped out of the screen as being very active:

The first active lead compound

As one can see the complete left hand side of Halichondrin has gone! However, this compound was not active in vivo. Many derivatives and analogues of this compound were prepared: furans, diols, ketones and so on and a lead emerged from this complex SAR study, ER-076349. The vicinal diol was used as a handle for further refinement and lead ultimately to E7389, the clinical candidate.

It can be synthesised in around 35 steps from simple starting materials.

Going through all this work in a few sentences really belittles the tremendous amount of effort that went into discovery and development of this compound and the people involved are to be applauded for their dedication.

Kishi continues to optimise the synthesis of Eribulin as judged by a recent publication10. Where he describes an approach to the amino-alcohol-tetrahydrofuran part of Eribulin (top left fragment, compound 1 below). The retro-synthetic analysis is shown below. The numbering corresponds to that of Eribulin.

The first generation synthesis consisted of 20 steps and delivered compound 1 about 5% yield, the second-generation route was completed in 12 steps with a yield of 48%. One of the highlights includes a remarkable asymmetric hydrogenation11 with Crabtree’s catalyst12:

This selectivity was not just luck; it seems to quite general, at least in this system. I always wonder how long it took them to stumble across this catalyst, but then I suppose that Eisai like most of the large pharma. companies has a hydrogenation group that probably indulges in catalyst screening.

The C34-C35 diol was obtained by a Sharpless asymmetric hydroxylation, here the diastereoisomeric ratio was not very high, only about 3:1 in favour of the desired isomer. Fortunately the undesired isomer could be removed completely by crystallisation.

This is a remarkable story and references 1 and 2 are worth reading to obtain the complete picture and learn lots of new chemistry as well. Eisai filed a NDA and the FDA approved the compound in 2010 for the treatment of metastatic breast cancer.

(-)-FR 182877 and (-) FR 182876 are two structures isolated from Streptomyces sp. No.9885 by researchers at the Fujisawa Pharmaceutical company (now Astelllas)1-5.

These compounds have been known for several years and the academic community has developed various approaches to their total synthesis, for example, references 6 and 7.

Nakada etal8 became interested in the unusual highly strained ring system in (-)-FR182877, shown below.

The picture beneath is a ChemDraw 3D energy minimised structure supposidly highlighting the strained nature of this ring system although I could not find an orientation which made it look good (my ineptitude with ChemDraw 3D).

Nakada became interested in the biological activity of and a synthetic approach to this bicyclic system8. This is a highly strained system owing to the distorted double bond, which is easily oxidised. The route he chose was the utilisation of an inverse electron demand Diels-Alder reaction. The retro-synthetic analysis leading to this approach is as follows: the red arrows are my attempt to indicate the retrosynthetic Diels-Alder reaction, with the blue bonds being those involved.

TES = triethylsilyl.

A brief digression into the realms of cycloaddition chemistry is now required. Diels and his student Alder carried out the seminal work in this area in 1928 and were awarded the Nobel Prize in 1950 for their efforts.

Basically you take a diene and a dienophile heat them and if you have chosen correctly out pops a nice new ring system, for example.

The diene is electron rich, and the dienophile electron deficient. This reaction has been studied in great detail throughout the years. Chirality control is easily achieved, as is regio- and stereo-selectivity, and I refer the reader to the many books and reviews published on this topic. One review outlining the use of the Diels-Alder reaction in natural product synthesis is especially worth reading9.

The Inverse Electron Demand Diels-Alder reaction is just the opposite of the above, i.e. electron poor diene, electron rich dienophile. For an interesting description of this reaction please go to the Wiki page, where it is wonderfully described. Substrates containing heteroatoms can also be employed in this reaction.

In the case I present here compound A was deemed to be a suitable substrate for this process, this, because, it was surmised that the HOMO and the LUMO were sufficiently near enough in energy this being brought about by proximity of the electron withdrawing ester group and the electron donating methyl moiety being in close proximity to one another.

Indeed heating compound A for 4 days produced the methyl ester of new ring system as a single isomer in 63% yield. According to Nakada this is the first example of this type of, inverse electron demand intramolecular hetero-Diels-Alder, reaction (IMHDA) to be reported.

However, he was still far away from the desired product and it required some chemical manoeuvring to get there. Deprotection of the methyl ester proved troublesome and they were unable to obtain the carboxylic acid, thus the methyl ester was replaced by a p-methoxybenyl ester (PMB), which is easily removed by employing DDQ, although they used trifluoroacetic acid which simultaneously removed the TES protection.

This ester exchange was not achievable after the IMHDA, which meant that they had go back to the start of the synthesis to introduce it, unfortunately a fact of life. However, they obtained the same Diels-Alder adduct in similar yield with the PMB ester.

Deprotection with TFA in water at 0°C produced the hydroxy acid. Treatment with Mukaiyama’s reagent10 delivered compound C presumably via the desired compound. As predicted the desired ring system was highly reactive and underwent addition of various nucleophiles across the strained double bond.

X-ray confirmed the structure of compound C. This new ring system proved to biologically inactive in a variety of cancer cell assays. A 1H-NMR spectrum of the desired compound was obtained from the crude product.

I think this is a marvellous example of the power of the Diels-Alder reaction in forming ring systems. I stand corrected, but I do not think there are many examples of this reaction employed by industry, especially the big pharma branch. This may be simply due to the nature of the set of investigational drugs currently being examined, which tend to be rather flat molecules with lots of nitrogen and may be better explosives than pharmaceuticals.

Postscript: after reading a very recent publication by Stössel etal in Organic Process Research and Development, 2012, DOI: 10.1021/op300145k. I can understand why the Diels-Alder reaction is not commonly seen in industry. This interesting analysis should be read by everyone as it highlights the dangers of uncontrolled chemical reactions with an actual example. It also demonstrates that these difficulties may be overcome. Indeed it is being employed in API production see, Stefan Abele, Stefan Höck, Gunther Schmidt, Jacques-Alexis Funel, and Roger Marti Org. Process Res. Dev., 2012, 16 (5), 1114–1120.

Following on from the tea party where polyphenols reared their (ugly) heads a “highlight” has appeared in Angewandte Chemie English edition1, 2 pointing out the benefits of red wine, i.e. resveratrol. This is a well-known molecule, which has been at the centre of some controversy of late. Resveratrol is chemically trans3,5,4’-trihydroxystilbene:

This compound can be found in many types of fruits and nuts berries AND grapes. Its concentration in red wines varies between 0.1 and 14 mg/L whereby the 3-glycosate achieves levels of 30 mg/L. Frequently associated with this compound is the “French Paradox”, not that the French are a paradox themselves, but that apparently, in spite of consuming large amounts of saturated fats and barrels of red wine, the incidence of heart disease is lower that one might well expect it to be3. Resveratrol has a plethora of biological activities associated with it:

It was originally noticed for its inhibitory effects against the oxidation of lipoproteins, the low-density variety being present at the onset of atherosclerosis4.

More about the sirtuins can be found on this page. In detail sirtuin1 information can be found here.

I do not profess to be conversant with all the details surrounding the apparent controversy concerning this compound and its biology. However, there is also big money at stake. GlaxoSmithKline acquired Sirtis, a company founded to discover and develop small molecules with at least some of the seven biological properties listed above. So presumably they are carrying out extensive medicinal chemistry on resveratrol. This won’t be an easy task to pick out one given the multitude of activities associated with this system, perhaps they can bundle 5,6 & 7 together. That is, of course, if there is any money left after paying the rather large fine recently dished out by the US Government. But, there is always the chance of off label indications being discovered!

Well, I shall certainly extend my red wine cellar but there won’t be many bottles in it, as I must take my daily dose of resveratrol by the bottle, especially at my age. Not to mention imbibing in tons of vitamin c and gallons of green tea. So when I drop dead after taking that lot no doubt I shall be considered as “toxic waste” and be treated accordingly.

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